1. Technical Field
The present invention relates to a power system, and more particularly, to a power-boosting system and method.
2. Discussion of the Related Art
FIGS. 1A and 1B illustrate voltages for driving a thin film transistor liquid crystal display (TFT LCD). For example, FIG. 1A illustrates a circuit model for driving a pixel in a TFT LCD, and FIG. 1B is a waveform showing the relationship among the voltages used in the circuit model of FIG. 1A. As shown in FIGS. 1A and 1B, a TFT is turned on by a voltage VGH output from a gate driver and is turned off by a voltage VGL output from the gate driver. A liquid crystal CLC and a storage capacitor CST are driven by common voltages VCOMH and VCOML that are periodically inverted. While the TFT is being turned on, pixel data represented by a voltage between VDH and VDL is stored in the liquid crystal CLC and the storage capacitor CST.
As shown in FIG. 1B, the voltage VGH that turns on the TFT is much larger than the voltage (between the voltages VDL and VDH) of the pixel data to rapidly charge the pixel data. In addition, the voltage VGL that turns off the TFT is much smaller than the voltage (between the voltages VDL and VDH) of the pixel data to prevent the occurrence of cross talk and reduce leakage current. To drive a TFT LCD, for example, a mobile video graphics array-class (VGA-class) TFT LCD, a TFT turn-on voltage of 20V and a TFT turn-off voltage of −20V are used. The TFT turn-on/off voltages vary with the type of TFT used in an LCD (e.g., an amorphous-silicon low temperature polysilicon (a-Si, LTPS), or continuous grain silicon TFT (CGS TFT)), and panel size. Because a voltage of about 3V is provided from a battery of a portable device, there is a need for a boosting circuit that steps the voltage of 3V up to 20V or down to −20V to drive the mobile VGA-class TFT LCD.
FIG. 2 is a block diagram of a conventional boosting voltage providing circuit. Referring to FIG. 2, the conventional boosting voltage providing circuit includes a reference voltage generating circuit 210, a buffer 220, a positive voltage boosting circuit 230 that uses first and second capacitors CA and CB, a positive/negative voltage boosting circuit 240 that uses a plurality of capacitors CC, CD, and CE1 through CE4, a positive voltage regulating and buffering circuit 250, and a negative voltage regulating and buffering circuit 260. In addition, the conventional boosting voltage providing circuit includes capacitors CL1, CL2, and CL3 that regulate output voltages.
In FIG. 2, the positive voltage boosting circuit 230 outputs a voltage DDVDH that is obtained by boosting an input voltage VCI1 twice by using the capacitors CA and CB. The positive/negative voltage boosting circuit 240 outputs a positive boosting voltage VLOUT2 that is four, five, or six times the input voltage VCI1 by boosting the voltage DDVDH using the plurality of capacitors CC, CD, and CE1 through CE4 or outputs a negative boosting voltage VLOUT3 that is minus three, minus four, or minus five times the input voltage VCI1 by dropping the voltage DDVDH using the plurality of capacitors CC, CD, and CE1 through CE4. The positive boosting voltage VLOUT2 and the negative boosting voltage VLOUT3 are regulated by the positive voltage regulating and buffering circuit 250 and the negative voltage regulating and buffering circuit 260 and are then output as a TFT turn-on voltage VGH and a TFT turn-off voltage VGL, respectively. Voltage regulators and unit gain buffers that are used in the positive voltage regulating and buffering circuit 250 and the negative voltage regulating and buffering circuit 260 are implemented as operational amplifiers.
FIG. 3 illustrates a conventional voltage boosting circuit. Boosting circuits 230 and 240 of FIG. 2 can be configured with the voltage boosting circuit of FIG. 3. As shown in FIG. 3, when a first switch SW1 and a second switch SW2 are closed and a third switch SW3 and a fourth switch SW4 are opened, the first capacitor CA is charged to a voltage VCI1. When the third switch SW3 and the fourth switch SW4 are closed and the first switch SW1 and the second switch SW2 are opened, the voltage DDVDH, which is two times the input voltage VCI1 by charge pumping, is output through the second capacitor CB.
The conventional boosting voltage providing circuit of FIG. 2, however, uses the capacitors CL1, CL2, and CL3 for regulating output voltages and the capacitors CA, CB, CC, CD, and CE1 through CE4 for voltage boosting. The large number of passive elements mounted in a module, which includes the conventional boosting voltage providing circuit of FIG. 2, outside an LCD panel, for example, increases the failure rate, and the volume of the module. Thus, the conventional boosting voltage providing circuit of FIG. 2 is not conducive for mobile products requiring compactness and lightweight. In addition, in the conventional boosting voltage providing circuit of FIG. 2, the boosting circuits 230 and 240, which are operated by a dynamic switching circuit that operates with a predetermined clock signal, constantly operate without regard to the amount of power used by an LCD panel, thus consuming considerable power during operation. As a result, the conventional boosting voltage providing circuit of FIG. 2 is typically unfavorable for use in mobile products requiring low power consumption.